The present invention relates to time of flight (TOF) measurement systems. More particularly, the present invention relates to a ground penetrating radar (GPR) system that is capable of providing A-scan images of subsurface targets using a synthetic aperture, end-fire array.
In the past, GPR has been used for a number of diverse applications, for example, geophysical applications such as mapping subsurface strata; locating toxic waste sites for remediation; and detection and location of unexploded subsurface ordnance.
GPR systems are similar to ordinary radar systems in that both measure target range (i.e., the distance from the radar system to an intended target, or portion thereof) by determining the amount of time it takes for electromagnetic (EM) radiation to travel from the radar to the intended target and then back to the radar. In practice, however, conventional GPR systems are inherently more complicated than ordinary radar systems due to some unique problems associated with transmitting and receiving EM radiation through a subsurface medium.
The first problem is that the subsurface medium (e.g., the earth) is typically inhomogeneous. Therefore, the EM radiation in a GPR system must travel through a number of different media, for example, air, rock, sand, water, clay, and other types of subsurface mineral deposits, each with a different and unquantified dielectric constant. Hence, the propagation velocity of the EM radiation from point to point within the subsurface volume may vary dramatically and is typically unknown without first performing a detailed, time-consuming analysis of the subsurface volume.
Ordinary radars do not encounter this problem because they transmit and receive EM radiation through "free space" (i.e., air) which is a homogeneous medium with a known dielectric constant. Because the dielectric constant of air is known, the propagation velocity of the EM radiation traveling through the air is known. Therefore, the computation of target range is quickly reduced to the task of multiplying the EM radiation time-of-flight (i.e., the round trip travel time between the radar and the target) by the propagation velocity of EM radiation through air.
The second problem associated with conventional GPR is that EM radiation does not penetrate the earth as easily as it penetrates the air. In fact, some media, such as wet clay or salt water, are so absorbent that EM radiation, at the frequency ranges relevant to GPR, cannot penetrate more than a few inches. The ability to penetrate a subsurface medium is highly dependent upon the frequency of the EM radiation. More specifically, low frequencies tend to achieve greater subsurface penetration. Unfortunately, lower frequencies also result in decreased target range resolution (i.e., target range accuracy). However, range resolution is also dependent upon bandwidth. More recently, GPR systems have begun employing ultra-wideband techniques, especially ultra-wideband impulse techniques which, to some extent, improve a GPR's ability to penetrate a subsurface medium without sacrificing resolution.
Although the two above-identified problems are by no means the only problems that affect GPR performance, they are clearly two very significant problems. Regarding the problem of subsurface inhomogeneity, some GPR systems employ a brute force technique that involves determining the propagation velocities for each region in the subsurface volume. However, as one might expect, these systems tend to be unacceptably slow. Consequently, there is a need to produce a GPR system that, despite the above-identified problems, can produce a subsurface image in real or near real-time. Moreover, there is a need to provide such a system that is physically compact so that it can be utilized in a spatially limited area.